Aqueous resin dispersions are widely employed as coatings, adhesives and textile assistants. From the standpoint of environmental conservation and workability, aqueous resin dispersions have been replacing solvent-type resin compositions.
However, containing polymers having a molecular weight as high as 500,000 to 1,000,000 in a particulate form, the conventional aqueous resin dispersions have insufficient fluidity for obtaining sufficient penetrability into a substrate and sufficient adhesion to a substrate. Additionally, a coating film having a smooth surface cannot be obtained.
In order to overcome these disadvantages, reduction of resin particles in size to prepare a so-called microemulsion or addition of a water-soluble resin has been attempted. However, reduction of dispersed particle size of an emulsion requires a large quantity of an emulsifying agent, which leads to reduction of the water resistance of the resulting film. Addition of a water-soluble resin also results in reduction in water resistance of a film to an impractical level.
An aqueous dispersion of a carbonyl-containing copolymer resin which contains a hydrazine derivative is known to provide a film having excellent water resistance, solvent resistance and hardness (see U.S. Pat. Nos. 4,267,091, 4,210,565, 5,208,282 and 4,959,428 and JP-B-1-13501 (the term "JP-B" as used herein means an "examined published Japanese patent application")). Nevertheless, the proposed aqueous resin dispersion is still inferior in adhesion to a substrate and surface smoothness of a film because the polymer exists in a particulate form.